Heretofore, for thickening cosmetics, there is known to use as a thickener a polysaccharide such as xanthane gum, a hydrophilic synthetic polymer such as polyacrylic acid, and a clay mineral such as bentonite or the like.
However, when using a polysaccharide such as xanthane gum or the like as a thickener in a system containing both a pharmaceutical ingredient and a salt, although it exhibits excellent stability of the system, but is suffers a disadvantage of providing a sticky feeling in use. When in using a hydrophilic synthetic polymer such as polyacrylic acid or the like as a thickener, it exhibits good feeling in use of non-stickiness and freshness; however, if a pharmaceutical ingredient and a salt are highly-incorporated into a system, it occurs a viscosity decrease of the system as the salt tolerance and the ion tolerance thereof are poor. Further, when in using a clay mineral such as bentonite or the like as a thickener, it suffers a disadvantage of providing a feeling of squakiness in use
On the other hand, as a water-soluble thickener having an excellent thickening effect, a thickener consists of a hydrophobic denatured polyether urethane (associative polymer) has been developed (Patent Reference 1), and is used in cosmetics (Patent References 2 and 3). That is to say, Patent Reference 2 discloses a cosmetic composition containing a hydrophobic denatured polyether urethane, and a carboxyvinyl polymer and/or xanthane gum; and Patent Reference 3 discloses an external preparation for skin containing a hydrophobic denatured polyurethane and collagen.
Furthermore, as a thickener having a non-stickiness and giving a fresh feeling in use, a microgel has been developed, which is prepared by grinding or crushing a hydrophilic compound gel having a gelling ability, and is used in cosmetics (Patent Reference 4). Other one than the above-mentioned microgel, another type of microgel has been separately developed through radical polymerization of a water-soluble polymerizable monomer; and a thickening composition as well as a cosmetic preparation containing the said radical-polymerized microgel and a hydrophobic denatured polyether urethane is disclosed (Patent Reference 5).
However, each of the compositions and the cosmetics disclosed in the above-mentioned Patent References 2 to 5 is for the purpose of preparing a base having a relatively high viscosity such as a cream-like base or the like. The composition of Patent Reference 2 has a thickening effect and good feelings in use, such as moistness and non-stickiness, but it has not been investigated in point of the viscosity stability thereof in a low to moderate-level viscosity range and of the fresh feeling in use. Also the composition of Patent Reference 3 has a thickening effect and a good feeling of elasticity in use, but has not been investigated in point of the viscosity stability thereof in a low to moderate-level viscosity range and of the fresh feeling in use. The thickener of Patent Reference 4 has a thickening effect and a fresh feeling in use, but has not also been investigated in point of the viscosity stability thereof in a low to moderate-level viscosity range and of the blend into skin feeling thereof. The composition of Patent Reference 5 has a thickening effect, and a new-type feeling (new-type elastic feeling) effect, but has not also been investigated in point of the viscosity stability thereof in a low to moderate-level viscosity range.